PURPOSE: To maintain the stability of a sensitizer and to prevent the sensitizing activity from being deteriorated by specifying the mixing ratio between palladium salt and stannous salt.

CONSTITUTION: This sensitizer is obtd. by mixing Pd and Sn (II) in 1:30W1:60 molar ratio in an acidic aqueous soln. and heating them. At this time, by raising the temp. to about 80W120°C after the mixing, both the sensitizing activity and stability are enhanced furthermore. For example, about 730ml pure water, about 200ml concd. hydrochloric acid, about 3g PdCl2, about 210g SnCl2 and about 0.5g surfactant are heated, boiled for about 1W2hr, and cooled slowly. The resulting soln. is taken out by about 50ml and added to about 1l hydrochloric acid with about 10% concn. to prepare a sensitizing soln. No Pd is settled in the soln., and superior stability is shown.